1. Field of the Invention
The present invention relates to a thermal field emission electron gun used for an electron beam machine such as an electron microscope, an electron beam machine for measuring the length of semiconductors, an electron beam lithograph machine, an electron beam tester or the like.
2. Discussion of Background
In recent years, in order to obtain an electron beam machine of low acceleration voltage requirements, a high resolution and a high throughput, an electron source capable of supplying stably a high current density at a low voltage has been required. For this, there has been used a thermal field emission electron gun comprising a thermal field emission cathode and a suppressor electrode wherein the cathode has a single crystal tungsten needle having an axis of direction of &lt;100&gt; which is covered with a coating layer comprising zirconium and oxygen, for reasons that it has a high brightness and a long service life in comparison with a conventional thermal field emission electron gun, and it operates more stable than a cold field emission electron gun.
In the conventional thermal field emission electron gun, molybdenum was used for the suppressor electrode (a pamphlet "Thermal Field Emitter Operating Instructions, Jan., 1987" published by FEI Company). However, the surface of molybdenum was easily oxidized and the oxide of molybdenum was apt to absorb gas. Accordingly, there were problems that the degree of vacuum was deteriorated in the use of the thermal field emission electron gun; the occurrence of an electric discharge might break it, or the rising time of emission was long.
For this problems, the material for the suppressor electrode has been studied.
For instance, Japanese Unexamined Patent Publication No. 94237/1990 discloses use of stainless steel which suppresses discharging of gas and prevents the breakage of the gun due to an electric discharge phenomenon.
Further, Japanese Unexamined Patent Publication No. 12973/1994 discloses that material having a diffusion coefficient of hydrogen gas smaller than a predetermined value is used for the suppressor electrode, and a stable rising time is obtainable without producing the electric discharge phenomenon by operating the gun under predetermined conditions of operation.
However, even in the disclosed methods, electron beams could not be stably obtained in a short time.